Recent structural and ultrastructural investigations have estimated that more than 90% of the neurons in the caudate nucleus are intrinsic or local circuit neurons, that is their dendrites and axons terminate within the caudate and interconnect with other neurons in the caudate (Kemp 1968a and b; Kemp and Powell 1971a,b,c,d,e). Our electrophysiological recordings of single unit responses of caudate neurons to electrical pulses closely applied within the intact or isolated caudate confirm the above anatomical investigations and demonstrate that the population of caudate neurons establishing synaptic connections with other caudate neurons is very large (Marco et al, 1972, 1973a and b). Although the dopaminergic identity of the nigro-neostriatal pathway has been established (Hornykiewicz, 1971), little is known about the neurotransmitter identity of intrinsic caudate neurons. We propose to investigate the responsiveness of receptor sites of caudate neurons to various putative neurotransmitter substances such as dopamine, norepinephrine, acetylcholine, serotonin, and gamma-amino-butyric acid. This research will be conducted in young cats in which one of the two caudate nuclei has been chronically isolated from the rest of the neuraxis. At a second acute stage, single unitary recordings of the previously isolated and of the intact caudate will be taken by means of five-barreled glass micropipettes. We will characterize unitary responses to a) electrical pulses delivered to closely apposed tungsten microelectrodes according to a procedure described in detail elsewhere (Marco et al, 1973a, b), b) to distant (cortex, thalamus, substantia nigra) stimulating electrodes, and c) responses to iontophoresis of putative neurotransmitter substances delivered to the immediate vicinity of the recorded units.